This invention relates generally to carburetors and more particularly to diaphragm type carburetors.
Typically, carburetors have been used to supply a fuel and air mixture to both four stroke and two stroke internal combustion engines. For many applications where small two stroke engines are utilized, such as hand held power chainsaws, weed trimmers, leaf blowers, garden equipment and the like, carburetors with both a diaphragm fuel delivery pump and diaphragm fuel metering system have been utilized. To start an engine which has its fuel supplied through such a carburetor, a purge system must be actuated several times to remove air from the carburetor and to fill desired fuel passages and chambers with liquid fuel to facilitate starting the engine. A choke valve must also be moved to its closed position at least substantially preventing air flow through a throttle bore of the carburetor. Thereafter, the engine starter mechanism must be actuated at least once and usually several times until a xe2x80x9cfalse startxe2x80x9d event occurs. This false start event occurs when combustion occurs and the engine initially turns over or starts but ceases to operate or dies soon thereafter due to the closed choke valve which prevents a sufficient air flow to the engine for its continued operation. After the false start event, the choke valve is manually moved to its open position and the starter mechanism again actuated to start the engine and provide sufficient air to the engine for its continued operation after it is started.
This multi-step starting procedure is somewhat complicated and difficult for the average consumer to easily and properly perform. In particular, many consumers have difficulty recognizing the false start event and therefore, they attempt to start the engine after the false start event with the choke valve closed. With the choke valve closed, as previously mentioned, there is an insufficient air supply to the engine for its continued operation and hence, even if the engine is initially started by this procedure it soon thereafter ceases to operate.
Some conventional carburetors utilize a priming system which injects a small quantity of liquid fuel into the throttle bore in response to the depression of a separate button on the carburetor to discharge the liquid fuel into the throttle bore. The liquid fuel provided into the throttle bore is desirable to provide a richer fuel and air mixture to facilitate starting the engine.
A diaphragm type carburetor which has a purge system to remove air and fuel vapor from the carburetor and to fill it with liquid fuel prior to starting an engine fed by the carburetor. The system has a primer valve actuated by moving a choke valve of the carburetor to its closed position to inject a small volume of liquid fuel into a throttle bore to provide a richer fuel and air mixture to the engine to facilitate starting it. Desirably, the small volume of liquid fuel is injected into the throttle bore in response to closing of the choke valve without requiring any additional steps such as depressing a button on the carburetor, to do so. Preferably, the choke valve is carried on a shaft which has a cam surface which actuates the primer valve as the choke valve is rotated from its open position to its closed position. Preferably, the choke valve shaft also has a pair of detents which close the primer valve and releasably retain the choke in its open and closed positions respectively. The primer valve is partially received in a recess of one detent when the choke valve is fully opened to close the primer valve and prevent any fluid flow therethrough. A recess of a second detent partially receives the primer valve to also close the primer valve after the choke valve has been moved to its fully closed position. The camming surface is disposed between the two detent recesses relative to the movement of the choke valve to temporarily open the primer valve and inject the desired quantity of liquid fuel into the throttle bore to assist starting the engine when the choke valve is moved between its open and closed positions.
Desirably, a purge system of the carburetor draws fuel from a fuel tank into a primer reservoir which has an overflow outlet leading back to the fuel tank and a diaphragm biased by a spring tending to reduce the volume of the primer reservoir and constructed to pressurize the fuel therein. Therefore, when the primer valve is opened as the choke valve is moved to its closed position, the fuel under pressure in the primer reservoir rushes past the temporarily opened primer valve and is injected into the throttle bore.
Objects, features and advantages of this invention include providing a carburetor which facilitates starting the engine, provides a quantity of liquid fuel into a throttle bore of the carburetor before starting of the engine to prime the carburetor, is self priming when the choke valve is moved to its closed position without requiring any additional steps during the starting procedure, maintains the primer valve closed when the choke valve is in its open and closed positions to prevent flooding the engine even if the air purge system of the carburetor is actuated after the choke valve is closed, permits a sufficient air flow through the throttle bore even when the choke valve is closed to permit sustained operation of the engine after initial starting even with the choke valve closed, is of relatively simple design and economical manufacture and assembly, is reliable, durable and has a long in service useful life.